This invention relates generally to a dual voltage tandem engine start system and method, and more specifically to a dual voltage tandem engine start system and method for starting hybrid electric vehicles.
Hybrid electric vehicles achieve high fuel mileage and low vehicle emissions by combining a battery-powered electric motor/generator (MG) with a highly efficient heat engine, typically an internal combustion engine (ICE). By using on-board engine computer controls to vary when each engine or combination of engines are used, the hybrid motor vehicle can achieve peak efficiency in different driving conditions. Hybrid motor vehicles can be classified as xe2x80x9cseriesxe2x80x9d or xe2x80x9cparallelxe2x80x9d hybrid motor vehicles. Series hybrids only have the electric motor/generator powering the driving wheels, while the heat engine drives a generator that serves to recharge the electric motor""s battery. Parallel hybrids use both the electric motor/ generator and the heat engine to provide power to the driving wheels. Most parallel hybrid vehicles do not fix the ratio of power from the electric motor/generator and the heat engine, but rather vary the ratio of power from the electric motor/generator and heat engine depending on which engine or motor or combination thereof has the greatest efficiency in a particular situation.
Parallel hybrid motor vehicles can be further classified as either xe2x80x9csoftxe2x80x9d (often also known as xe2x80x9cmildxe2x80x9d) or xe2x80x9cfull.xe2x80x9d Soft or mild hybrids always operate with the heat engine powering the driving wheels, while the electric motor/generator provides extra power to the wheels in parallel with the heat engine""s power only when needed. In contrast, full hybrids do not always have the heat engine powering the driving wheels. Depending on the power required at the driving wheels, the vehicle control unit will select between only the electric motor/generator driving the wheels, only the heat engine driving the wheels, or a combination of both electric motor/generator and heat engine driving the wheels in parallel. Typically, a full hybrid motor vehicle will utilize the electric motor/generator alone to power the driving wheels from a stop (such as a stoplight) or under light loads, such as in stop-and-go traffic; that is, under conditions in which the electric motor/generator can perform more efficiently at varying speeds and motor rotational speeds than can the heat engine. At constant loads, such as when cruising at a steady speed and RPM on the highway, the heat engine alone will power the driving wheels. When the full hybrid motor vehicle is under heavy load, such as when it is accelerating rapidly or driving up an incline, both the electric motor/generator and the heat engine will provide power to the driving wheels. Most parallel hybrid vehicles, whether they are full or soft hybrids, will turn off the heat engine when the car is stopped for an extended amount of time (for example, at a stoplight) to conserve fuel. The electric motor/generator, as well as the motor vehicle""s electrical system, will remain on. As soon as the accelerator pedal is depressed, the heat engine will immediately start up (in the case of a soft hybrid) or will remain off until needed, with only the electric motor/generator powering the driving wheels (in the case of the full hybrid). Hybrid motor vehicles thus can achieve the same efficiency as a motor vehicle with a small engine, but without sacrificing undue amounts of power and engine performance.
Hybrid motor vehicles differ from conventional, fully electric vehicles by not requiring that the vehicle be xe2x80x9cplugged inxe2x80x9d to recharge the motor/generator""s battery pack. Hybrid motor vehicles recharge their battery packs in the process of driving and, barring a dead battery or a system failure, do not require any outside battery charging. This battery recharging is done by the motor/generator in combination with the heat engine. The motor/generator is built in such a way that it functions as a motor, turning a crankshaft or driveshaft, when supplied with power, and functions as a generator, producing electric power, when the motor/generator""s crankshaft or driveshaft is turned by some outside force. Thus, when the heat engine turns the motor/generator""s crankshaft through a belt drive, system of gears, or the like, the motor/generator produces electricity that recharges the battery or battery pack. Most hybrid vehicles also utilize regenerative braking, in which the electric motor/generator is turned by the driveshafts connected to the driving wheels as the motor vehicle coasts to a stop, allowing the motor/generator to turn and recharge the battery or battery pack.
In most hybrid motor vehicles, the motor/generator, battery pack, and related hardware are designed to operate at a higher voltage than the voltage used throughout the rest of the motor vehicle. That is, hybrid vehicles retain the conventional 12-volt battery and related electronic equipment found in non-hybrid motor vehicles to run the hybrid motor vehicle""s accessories (such as the headlights, radio, electronic componentry, engine computer, and the like) so as to keep the hybrid motor vehicles as unchanged as possible from conventional motor vehicles and thus cut down on manufacturing costs. Hybrid motor vehicles also utilize a separate, higher voltage motor/generator bus to power the driving wheels. There are a number of reasons for using a higher voltage motor/generator: it allows the engine to be restarted more quickly than is possible with a conventional 12-volt system, and it provides more power to the driving wheels when accelerating, hill climbing, and the like. The way in which the higher voltage system and the traditional 12-volt system are configured varies between different hybrid motor vehicle models. The most notable differences between dissimilar configurations are the way in which they perform the hybrid motor vehicle""s xe2x80x9cfirst start.xe2x80x9d xe2x80x9cFirst startxe2x80x9d refers to the motor vehicle being started for the first time after the motor vehicle has been completely shut off. xe2x80x9cFirst startxe2x80x9d does not refer to the motor vehicle""s heat engine being started after a long stop, for example at a stoplight. Such starts that occur after a delayed stop (such as at a stoplight) will be referred to hereafter as xe2x80x9cquick starts.xe2x80x9d It is desirable during the hybrid motor vehicle""s first start to spin the heat engine up to a sufficiently high rotational speed before providing the heat engine with fuel, so that when fuel is delivered to the heat engine, the heat engine steadily achieves stable combustion without any of the faltering associated with a lower rotational speed start. By utilizing this high rotational speed start, emissions for the first start are dramatically lowered over a conventional, non-hybrid heat engines first start, and fuel economy is improved.
One conventional configuration of the 12-volt system and the higher voltage system has the higher voltage system performing the hybrid motor vehicle""s first start while the 12-volt system is relegated to only powering the vehicle""s accessories. This system allows for a high rotational speed heat engine start, and thus low emissions, but leaves no option for jumpstarting the hybrid motor vehicle should the higher voltage system ever have insufficient charge, unless a large DC-DC converter is provided that connects the 12-volt and the higher voltage systems. As almost all motor vehicles on the road today are 12-volt, jumpstarting a higher voltage hybrid motor vehicle with a 12-volt system will not work without this large DC-DC converter. Oversized DC-DC converters are expensive and add a great deal of cost to the system. Also, for the higher voltage system alone to provide a cold first start, it must have a battery pack and inverter switches that are greatly oversized compared to inverter switches designed only to work in warm climates. Oversized battery packs and inverter switches are also very expensive.
One alternative to an all-higher voltage engine starting system provides for a backup 12-volt starter motor that is used to start the heat engine only in emergency situations when the higher voltage system has insufficient battery charge. In normal operation, when the hybrid motor vehicle""s batteries are all sufficiently charged to start the heat engine, only the higher voltage system provides a high RPM first start. If the higher voltage system""s battery or battery pack is insufficiently charged, but the 12-volt battery is sufficiently charged, then the 12-volt battery can be used to start the heat engine. If neither the 12-volt battery nor the higher voltage battery or battery pack are sufficiently charged, the 12-volt battery can be jumpstarted or charged to the point where it can start the heat engine. This system is prone to failure, however, because starter motors must be used on a regular basis or they are prone to freeze up and fail. Thus, unless the backup 12-volt starter motor is used on a regular basis, it may not be operable in the emergency case of an uncharged higher voltage battery or battery pack.
Another electrical system configuration, similar to the previous one, has the 12-volt starter motor perform the heat engine""s first start, with only the higher voltage system performing any other heat-engine quick starts during subsequent operation of the motor vehicle. The 12-volt system only powers the motor vehicle""s accessories after the first start. Thus, the 12-volt starter motor is protected from freezing up from nonuse; however, utilizing only the 12-volt starter motor to perform the motor vehicle""s first start does not allow the heat engine to reach a high enough rotational speed before receiving fuel to achieve a low emission start.
It is desirable to have an electrical system configuration that provides high rotational speed heat engine start-ups and the low emissions associated with such a start, but at the same time has a reliable backup system that can start the motor vehicle should part of the electrical system ever fail. The entire electrical system should also avoid the use of oversized DC-DC converters, battery packs, or oversized inverter switches in order to keep manufacturing costs down. Accordingly, a need exists for a dual voltage tandem engine start system that overcomes the problems attendant with prior art start systems.